1. Technical Field
The present invention relates to wireless Local Area Networks (LANs), and more particularly, to an Orthogonal Frequency Division Multiplexing (OFDM) receiver used in a LAN and an OFDM method therefor.
2. Description
Wireless Local Area Network (LAN) systems wirelessly connect public LANs and private LANs, to provide convenience of information transmission to users of devices such as computers and mobile communication terminal. Orthogonal Frequency Division Multiplexing (OFDM) signals using high frequency bands are generally received/transmitted on multiplex carriers at a maximum transmission rate of 54 Mbps in a 5.4 GHz frequency band, as defined by the IEEE 802.11a standard. IEEE 802.11 defines various types of signals such as a Direct Sequence Spread Spectrum (DSSS) signal and a Complementary Code Keying (CCK) signal. An OFDM receiver that receives OFDM signals is described below.
FIG. 1 is a diagram of a signal standard of a general OFDM signal, wherein a preamble area of the OFDM signal, which is defined in the IEEE 802.11a standard, is illustrated in detail. Referring to FIG. 1, the preamble area of the OFDM signal consists of a short training symbol interval having 10 repeated patterns within a time period of at least 8 μs, and a long training symbol interval having one Guide Interval (GI) and two repeated patterns within a subsequent time period of 8 μs. The short training symbol interval consists of 16 samples of a signal, and the long training symbol interval consists of 64 samples of a signal. Symbol timing synchronization means finding a boundary between the short training symbol interval and the long training symbol interval.
In a conventional symbol timing synchronization method, cross-correlation and peak value detection are performed using the short training symbol or the long training symbol as a reference signal, to generate symbol clocks. Since a better cross-correlation property can be obtained when cross-correlation is performed using the longer training symbol as the reference signal, cross-correlation is generally performed using the long training symbol as the reference signal. However, the long training symbol interval is received after the short training symbol, which reduces a response speed of a whole system. Details for signal receiving operations of an OFDM receiver are disclosed in Europe Patent No. EP 1,126,673.
The conventional symbol timing synchronization method which performs cross-correlation using the long training symbol as the reference signal can obtain a good cross-correlation property, since an OFDM signal is subjected to cross-correlation through a correlator during the long time period of the long training symbol interval. However, the conventional method cannot measure fine frequency offsets and channel coefficients before symbol synchronization is achieved. As a result, the conventional method described above reduces a response speed of the entire system. Also, if another technique is added to prevent response speed reduction, the entire system becomes complicated.
Meanwhile, a method in which cross-correlation is performed using the short training symbol as a reference signal can increase the response speed of the system. However, in such a method, since seven parts (t1-t7) within the short training symbol interval are used for performing operations such as Auto Gain Control (AGC), according to a property of IEEE 802.11a receivers, only three parts (t8-t10) are used as valid time periods by the correlator, which deteriorates the cross-correlation property. Also, because the number of signal samples of the short training symbol is small, the short training symbol has a poor correlation property due to effects of noise, frequency offsets, etc., compared to the long training symbol.
Accordingly, it would be desirable to provide an orthogonal frequency division multiplexing (OFDM) receiver used in a wireless LAN system, capable of improving a cross-correlation property and achieving fast symbol synchronization, by performing first cross-correlation for an OFDM signal using a differential value of a short training symbol as a reference signal, performing second cross-correlation for an output value of the first cross-correlation, on the basis of an auto correlation value of the differential value of the short training symbol, and performing double peak-value comparison for an output value of the second cross-correlation.
It would also be desirable to provide an OFDM receiving method used for a wireless LAN system, capable of improving a cross-correlation property and achieving fast symbol synchronization, by performing first cross-correlation for an OFDM signal using a differential value of a short training symbol as a reference signal, performing second cross-correlation for an output value of the first cross-correlation, on the basis of an auto correlation value of the differential value of the short training symbol, and performing double peak-value comparison for an output value of the second cross-correlation.
According to one aspect of the present invention, OFDM receiver comprises a radio frequency (RF) module unit, an analog-to-digital (A/D) converter, a first differential cross-correlation unit, a second differential cross-correlation unit, a first peak detector, a second peak detector, a symbol clock generator, and an inverse fast fourier transform (IFFT) unit.
The RF module receives a radio transmission, and extracts and outputs an OFDM analog signal from a signal existing on an allocated channel.
The A/D converter samples and converts the OFDM analog signal to an OFDM digital signal.
The first differential cross-correlation unit performs first cross-correlation for the OFDM digital signal and a differential value DVTS of a short training symbol according to an OFDM standard (an expected differential value of a short training symbol), and outputs a first differential cross-correlation value Z1.
The second differential cross-correlation unit performs second cross-correlation for the first differential cross-correlation value Z1 and an auto correlation value ACVTS of the differential value DVTS of the short training symbol, and outputs a second differential cross-correlation value Z.
The first peak detector compares a previous first peak value Z(dmax*(i−1)), which is a peak value among a previous M (e.g., 16) sample values of the second differential cross-correlation value Z, with a present first peak value Z(dmax*(i)) which is a peak value among a following (subsequent) M (e.g., 16) sample values of the second differential cross-correlation value Z, in response to a second boundary detection signal P indicating that a second condition is satisfied, and outputs a first boundary detection signal P1 corresponding to a first condition.
The second peak detector compares a previous second peak value Z(dmax+1*(i−1)), which is the next sample value after the peak value among the previous M (e.g., 16) sample values of the second differential cross-correlation value Z, with a present second peak value Z(dmax+1*(i)), which is the next sample value after the peak value among the following (subsequent) M (e.g., 16) sample values of the second differential cross-correlation value Z, in response to the first boundary detection signal P1 indicating that the first condition is satisfied, and outputs the second boundary detection signal P corresponding to the second condition. The symbol clock generator determines the location dmax*(i−1) of the previous first peak value as a boundary between the short training interval and the long training interval, and generates a symbol clock SCLK synchronized to the location dmax*(i−1), when the first boundary detection signal P1 indicating that the first condition is not satisfied, or the second boundary detection signal P indicating that the second condition is not satisfied, is active.
The IFFT unit synchronizes the digital signal output from the A/D converter to the symbol clock SCLK, performs an IFFT for the synchronized signal, and outputs a digital symbol IFTS.
Beneficially, the cross-correlation performed to calculate the first differential cross-correlation value uses Equations (1) through (4) below:
                              P          k                =                              b            k                    ⁢                      ⅇ                          j              ⁡                              (                                                      2                    ⁢                                                                                  ⁢                    π                    ⁢                                                                                  ⁢                    Δ                    ⁢                                                                                  ⁢                                          fkT                      s                                                        +                                      θ                    0                                                  )                                                                        (        1        )                                          b          k                =                              ∑                          n              =              0                                      N              -              1                                ⁢                                    a              n                        ⁢                          ⅇ                              j                ⁢                                                                  ⁢                                                      2                    ⁢                    π                    ⁢                                                                                  ⁢                    nk                                    N                                                                                                    wherein Pk is a k-th signal sample representing the digital signal, bk is an ideal k-th signal sample, Ts is a sample interval, Δf is the frequency deviation of the received signal, θ0 is an initial phase value of the received signal, N is a point size of an IFFT, and an is a data symbol from a transmission side transmitted on a n-th sub-channel;R1(k)=b*k−1bk  (2)wherein R1(k) is the differential value of the short training symbol;T(d)=Pk+d−1P*k+d  (3)wherein d is a location of a time area, that is, any sampled time location; and
                                                                        Z1                ⁡                                  (                  d                  )                                            =                                                ∑                                      k                    =                    1                                    16                                ⁢                                                      T                    ⁡                                          (                      d                      )                                                        ⁢                                      R1                    ⁡                                          (                      k                      )                                                                                                                                              =                                                ∑                                      k                    =                    1                                    16                                ⁢                                                      (                                                                  P                                                  k                          +                          d                          -                          1                                                                    ⁢                                              p                                                  k                          +                          d                                                *                                                              )                                    ⁢                                      (                                                                  b                                                  k                          -                          1                                                *                                            ⁢                                              b                        k                                                              )                                                                                                                          =                                                ∑                                      k                    =                    1                                    16                                ⁢                                                      (                                                                  b                                                  k                          +                          d                          -                          1                                                                    ⁢                                              b                                                  k                          +                          d                                                *                                                              )                                    ⁢                                      (                                                                  b                                                  k                          -                          1                                                *                                            ⁢                                              b                        k                                                              )                                    ⁢                                      ⅇ                                          j                      ⁡                                              (                                                  2                          ⁢                                                                                                          ⁢                          π                          ⁢                                                                                                          ⁢                          Δ                          ⁢                                                                                                          ⁢                                                      fT                            s                                                                          )                                                                                                                                                    (        4        )            wherein Z1(d) is the first differential cross-correlation value.
Beneficially, the first condition is expressed mathematically as the following Equation:β*Z(dmax*(i−1))<Z(dmax*(i)),wherein β is an arbitrary coefficient, Z(dmax*(i−1)) is the previous first peak value, dmax*(i−1) is the location of the previous first peak value, Z(dmax*(i)) is the present first peak value, and dmax*(i) is a location of the present first peak value.
Beneficially, β is less than 0.5.
Beneficially, the first boundary detection signal becomes non-active if the first condition is satisfied, and becomes active if the first condition is not satisfied.
Beneficially, the second condition is expressed mathematically as the following Equation:γ*Z(dmax+1*(i−1))<Z(dmax+1*(i)),wherein γ is an arbitrary coefficient, Z(dmax+1*(i−1)) is the previous second peak value, dmax+1*(i−1) is a location of the previous second peak value, Z(dmax+1*(i)) is the present second peak value, and dmax+1*(i) is a location of the present second peak value.
Beneficially, γ is less than 0.35.
Beneficially, the second boundary detection signal becomes non-active if the second condition is satisfied, and becomes active if the second condition is not satisfied.
According to another aspect of the present invention, an OFDM receiving method of a wireless LAN system, comprises: receiving a sky wave, and extracting and outputting an OFDM analog signal from a signal existing on an allocated channel; sampling and converting the OFDM analog signal into a digital signal; performing first cross-correlation for the digital signal and a difference value of a short training symbol according to an OFDM standard, and outputting a first differential cross-correlation value; performing second cross-correlation of the first differential cross-correlation value and an auto correlation value of the difference value of the short training symbol, and outputting a second differential cross-correlation value; comparing a previous first peak value Z(dmax*(i−1)), which is a peak value among a previous M (e.g., 16) sample values of the second differential cross-correlation value, with a present first peak value Z(dmax*(i)) which is a peak value among a following (subsequent) M (e.g., 16) sample values of the second differential cross-correlation value, in response to a second boundary detection signal indicating that a second condition is satisfied, and outputting a first boundary detection signal corresponding to a first condition; comparing a previous second peak value Z(dmax+1*(i−1)), which is a next sample value after the peak value among the previous M (e.g., 16) sample values of the second differential cross-correlation value, with a present second peak value Z((dmax+1*(i)) which is a next sample value after the peak value among the following (subsequent) M (e.g., 16) sample values of the second differential cross-correlation value, in response to the first boundary detection signal indicating that the first condition is satisfied, and outputting second boundary detection signal corresponding to the second condition; determining the location dmax*(i−1) of the previous first peak value as a boundary of a long training symbol interval and a short training symbol interval according to the OFDM standard, and generating a symbol clock synchronized to the location dmax*(i−1), when the first boundary detection signal indicating that the first condition is not satisfied, or the second boundary detection signal indicating that the second condition is not satisfied, is active; and synchronizing the digital signal to the symbol clock, performing IFFT for the synchronized signal, and outputting a digital symbol.
Beneficially, the first cross-correlation performed to calculate the first differential cross-correlation value uses Equations (1) through (4) below:
                              P          k                =                              b            k                    ⁢                      ⅇ                          j              ⁡                              (                                                      2                    ⁢                                                                                  ⁢                    π                    ⁢                                                                                  ⁢                    Δ                    ⁢                                                                                  ⁢                                          fkT                      s                                                        +                                      θ                    0                                                  )                                                                        (        1        )                                          b          k                =                              ∑                          n              =              0                                      N              -              1                                ⁢                                    a              n                        ⁢                          ⅇ                              j                ⁢                                                                  ⁢                                                      2                    ⁢                    π                    ⁢                                                                                  ⁢                    nk                                    N                                                                                                    wherein Pk is a k-th signal sample representing the digital signal, bk is an ideal k-th signal sample, Ts is a sample interval, Δf is the frequency deviation of the received signal, θ0 is an initial phase value of the received signal, N is a point size of an IFFT, and an is a data symbol from a transmission side transmitted on a n-th sub-channel;R1(k)=b*k−1bk  (2)wherein R1(k) is the differential value of the short training symbol;T(d)=Pk+d−1P*k+d  (3)wherein d is a location of a time area, that is, any sampled time location; and
                                                                        Z1                ⁡                                  (                  d                  )                                            =                                                ∑                                      k                    =                    1                                    16                                ⁢                                                      T                    ⁡                                          (                      d                      )                                                        ⁢                                      R1                    ⁡                                          (                      k                      )                                                                                                                                              =                                                ∑                                      k                    =                    1                                    16                                ⁢                                                      (                                                                  P                                                  k                          +                          d                          -                          1                                                                    ⁢                                              p                                                  k                          +                          d                                                *                                                              )                                    ⁢                                      (                                                                  b                                                  k                          -                          1                                                *                                            ⁢                                              b                        k                                                              )                                                                                                                          =                                                ∑                                      k                    =                    1                                    16                                ⁢                                                      (                                                                  b                                                  k                          +                          d                          -                          1                                                                    ⁢                                              b                                                  k                          +                          d                                                *                                                              )                                    ⁢                                      (                                                                  b                                                  k                          -                          1                                                *                                            ⁢                                              b                        k                                                              )                                    ⁢                                      ⅇ                                          j                      ⁡                                              (                                                  2                          ⁢                                                                                                          ⁢                          π                          ⁢                                                                                                          ⁢                          Δ                          ⁢                                                                                                          ⁢                                                      fT                            s                                                                          )                                                                                                                                                    (        4        )            wherein Z1(d) is the first differential cross-correlation value.
Beneficially, the first condition is expressed mathematically as the following Equation:β*Z(dmax*(i−1))<Z(dmax*(i)),wherein β is an arbitrary coefficient, Z(dmax*(i−1)) is the previous first peak value, dmax*(i−1) is the location of the previous first peak value, Z(dmax*(i)) is the present first peak value, and dmax*(i) is a location of the present first peak value.
Beneficially, β is less than 0.5.
Beneficially, the first boundary detection signal becomes non-active if the first condition is satisfied, and becomes active if the first condition is not satisfied.
Beneficially, the second condition is expressed mathematically as the following Equation:γ*Z(dmax+1*(i−1))<Z(dmax+1*(i)),wherein γ is an arbitrary coefficient, Z(dmax+1*(i−1)) is the previous second peak value, dmax+1*(i−1) is a location of the previous second peak value, Z(dmax+1*(i)) is the present second peak value, and dmax+1*(i) is a location of the present second peak value.
Beneficially, γ is less than 0.35.
Preferably, the second boundary detection signal becomes non-active if the second condition is satisfied, and becomes active if the second condition is not satisfied.